The present invention relates to mass flow measurements and more particularly to an apparatus and method for determining mass flow from a single measurement signal. In most mass flow measurements it is necessary to produce a signal related to the rate of fluid flow and a separate signal related to the density of the fluid and then combine the signals to produce a mass flow signal. The difficulty in all of these mass flow measuring systems is the ability to accurately measure the density of the fluid. Also all of these systems require the use of two separate means for producing signals, the first producing a signal related to the fluid flow and the second producing a signal relating to the fluid's density. This is an additional complication which increases the chances of malfunctioning or inoperativeness of the systems.
In recent years the Von Karman vortex shedding phenomena has been used to develop instruments for measuring fluid flow rates. The vortex shedding flow meter comprises a bluff body which projects into the fluid at substantially right angles to the fluid flow. As the fluid flows past the bluff body and separates from the bluff body on the downstream side, a vortex or turbulence is created. The frequency of oscillation of the vortex at right angles to the flow is directly proportional to the flow velocity and inversely proportional to the diameter of the object. The bluff body can take the shape of a cylinder or a triangular-shaped element with the base of the triangle facing the direction of flow. The advantage of a vortex shedding flow meter results from the fact that it is independent of specific gravity, viscosity, pressure or temperature of the fluid. Thus, the vortex generating object, if correctly shaped and placed in a pipeline or conduit, forms a primary flow measuring element that will generate pulse signals over wide flow ranges at a frequency proportional to the flow.
The fluid flow can be determined from the equation v= (fD)/s, wherein v is the velocity of flow, f is the vortex frequency, D is the diameter of the bluff body and s is the Strouhal number which remains constant over a very wide range of Reynolds numbers.
The weakness in the present vortex shedding meters is the method used for sensing the frequency of the vortex shedding. The primary method used in the state-of-the art relies on heated thermistors placed on opposite sides of the bluff body so that the thermistors will be unevenly heated as the vortex tends to oscillate back and forth in the fluid stream. While thermistors successfully detect the frequency of the vortex shedding, they are delicate devices and become damaged in most fluid systems. Thus, the advantages of the vortex shedding flow meters are not fully realized due to the malfunctioning of the sensing systems.